Centrifugal separator and method of operating same

ABSTRACT

In a centrifugal separator, the rotor of which has a separation chamber (7) with an inlet (16) for a liquid mixture, a constantly open central outlet (23) for a separated liquid and an intermittently openable peripheral outlet (13) for separated solids, a so-called displacement liquid is supplied to the separation chamber (7) before the peripheral outlet (13) is opened first at a certain flow rate and then at a substantially lesser flow rate. If displacement liquid is sensed in the separated liquid leaving the rotor through the central outlet (23), the peripheral outlet (13) is opened.

This invention relates to centrifugal separators and concernsparticularly a method of operating a centrifugal separator with a rotorhaving a separation chamber, which chamber has an inlet for a liquidmixture, a central outlet for separated liquid and a peripheral outletfor separated solids, opening means for opening and closing theperipheral outlet during operation of the rotor, supply means supplyingthe separation chamber with a predetermined amount of displacementliquid that is heavier than said separated liquid, each time theperipheral outlet is to be opened, and control means for activating thesupply means to supply displacement liquid immediately before theperipheral outlet is to be opened. The invention also concerns acentrifugal separator for performing said method.

In a centrifugal separator of the kind described , used in applicationswhere the incoming liquid mixture contains, apart from solids, two kindsof liquids to be separated, the separation chamber has three separateoutlets; two constantly opened outlets for the liquids and oneintermittently openable for the solids. A centrifugal separator of thiskind is described in U.S. Pat. No. 4,343,431. Lubricating oils, as anexample, are usually purified by means of centrifugal separators of thiskind.

A problem in connection with centrifugal separators of this kind is thatthey are sensitive to variations in temperature and flow rate of thesupplied liquid feed mixture. Upon such variations an interface layerformed within the separation chamber between the separated liquids movesradially inwards or outwards, for which reason it is difficult todetermine exactly its position in the separation chamber. Anotherproblem is to make the right choice, for a centrifuge rotor of thiskind, of the so-called gravity disk, the size of which determines thedesired radial level for said interface layer. Changing the diskrequires disassembling the rotor.

A centrifuge rotor with two constantly open liquid outlets has thefurther drawback that a large part of its separation chamber has to befilled with separated liquid, even if the content of this liquid in thesupplied mixture is very small or sometimes even zero. This means thatthe separation chamber is used inefficiently.

In a centrifugal separator of this general class the separation chambermay alternatively have only two outlets; one constantly open outlet forseparated, relatively light, liquid and one intermittently openableperipheral outlet for separated, relatively heavy solids. A centrifugalseparator of this kind is used principally when a liquid is to be freedonly from solids.

However, a centrifugal separator of this two outlet kind may be usedeven if the feed mixture contains two liquids in addition to solids. Ina case like this separated, relatively heavy, liquid is removedintermittently from the rotor together with separated solids through theperipheral outlet of the separation chamber. In such cases therelatively heavy liquid usually constitutes only a small part of thesupplied mixture.

A problem in using a centrifugal separator having only one centralliquid outlet, when the supplied mixture contains two different kinds ofliquids, is that the need for intermittent discharge of separated solidsusually does not coincide in time with the need for intermittentdischarge of separated, relatively heavy, liquid. This problem isparticularly difficult to solve if the content of heavy liquid andsolids, or either, in the feed varies. Further, it is difficult even ina centrifugal separator of this kind to determine exactly the radialposition of the interface layer formed in the separation chamber betweenthe separated liquids.

Regardless of whether a centrifugal separator has one or two constantlyopen liquid outlets, a so-called displacement liquid normally has to besupplied to the separation chamber just before each opening of theperipheral outlet for solids. The purpose of a supply of displacementliquid to decrease the amount of separated light liquid in theseparation chamber so that no such light liquid leaves the separationchamber through the peripheral outlet when it is opened. If the heavierof the liquids to be separated is water, water is normally used as thedisplacement liquid.

In this connection it has proved difficult to supply the optimum amountof displacement liquid, because this depends upon the problem describedabove of safely determining the radial position of the interface layerin the separation chamber between the two separated liquids. It willthus be uncertain how much displacement liquid may be supplied withoutit beginning to leak out through the outlet for separated light liquid.

The object of the present invention is to provide a method by which thisdifficulty can be avoided, so that a centrifugal separator of the kinddescribed may be used without risk of having an uncontrolled amount ofdisplacement liquid flowing out through the outlet for the separated,relatively light, liquid.

This object is achieved by supplying a first part of the displacementliquid at a predetermined flow rate, after which a second part of thedisplacement liquid is supplied at a substantially smaller flow rate;detecting the presence of displacement liquid in the separated liquidleaving the rotor through the central outlet; and opening the peripheraloutlet upon detecting displacement liquid in the separated liquid.

In this way it is possible, at each occasion when the peripheral outletof the separation chamber is to be opened, to supply an optimum amountof displacement liquid to the separation chamber, a small and controlledamount thereof being allowed to flow out through the outlet for theseparated, relatively light, liquid for determining the radial positionin the separation chamber of the interface layer between displacementliquid and separated, relatively light, liquid.

The invention presumes that the separation chamber contains at least acertain known amount of separated, relatively light, liquid, i.e., thatthe separating operation is under sufficient control that an interfacelayer between the separated light liquid and a separated heaviercomponent surely has not had time to move inside a certain radial levelin the separation chamber. The invention thus presumes that duringnormal operation of the centrifugal separator the first part of thedisplacement liquid may be supplied relatively rapidly without risk ofhaving a large part pass out through the outlet for the separated lightliquid. Because part of the displacement liquid may be suppliedrelatively rapidly, i.e., with a relatively large flow rate, the timeduring which the separation chamber has to contain displacement liquidand therefore cannot be used effectively may be kept as short aspossible. Because the second part of the displacement liquid is suppliedsubstantially more slowly, i.e., with a substantially smaller flow rate,mixing of a large amount of displacement liquid with the separated theseparated light liquid before appearance of displacement liquid in thedischarging light liquid is sensed, may be prevented. It has beendetermined that even if detection of displacement liquid in theseparated light liquid results in a very rapid opening of the peripheraloutlet of the separation chamber, the content of displacement liquid inthe separated light liquid which leaves the separation chamberimmediately after such an opening operation, becomes unacceptably high,if the displacement liquid is supplied at a too large a flow rate overthe entire course of its supply.

Preferably, according to the invention, the predetermined amount ofdisplacement liquid is supplied batch-wise and after the supply of onebatch, the next batch is supplied only after the result of the precedingbatch has been sensed in the separated liquid. It has been determinedthat when the interface layer between displacement liquid and separatedlight liquid has reached a certain critical radial level in theseparation chamber, it still takes a certain time from the moment when abatch of displacement liquid has been supplied to the moment whendisplacement liquid is detected in the separated light liquid leavingthe separation chamber. By batch-wise supply of displacement liquid itis possible to avoid mixing an unnecessarily large amount ofdisplacement liquid with separated light liquid accompanying it throughthe outlet. Preferably the first part of the displacement liquid issupplied substantially continuously in a relatively large batch, afterwhich the rest is supplied in smaller batches or increments.

The invention will be further described in the following with referenceto the accompanying drawings in which:

FIG. 1 shows a sectional view of a centrifuge rotor and, schematically,parts of the equipment for operation thereof in accordance with theinvention.

FIG. 2(a) and 2(b) are graphs illustrating two different methods ofsupplying displacement liquid according to the invention.

FIG. 1 shows a centrifugal separator comprising a rotor with an upperpart 1 and a lower part 2, which parts are held together by means of alocking ring 3. The rotor is supported from below by a vertical driveshaft 4. Within the rotor there is an axially moveable slide 5, which inits upper position, shown in the drawing, abuts against an annulargasket 6 arranged in a groove in the upper rotor part 1. In this upperposition the slide 5 with the upper rotor part 1 defines a separationchamber 7 within the rotor. Below the slide 5 and between it and thelower rotor part 2 is a so-called closing chamber 8 for a closing liquidfurnished during operation of the rotor through a stationary pipe 9, agroove 10 and a channel 11 in the rotor part 2.

At the peripheral portion of the rotor the rotor part 2 has a throttleddraining channel 12 extending from the closing chamber 8 to the outsideof the rotor body.

During operation of the rotor enough closing liquid is constantlysupplied to the closing chamber 8 so that the latter is kept filled. Theslide 5 is therefore maintained in its upper position, as shown, inwhich it closes the separation chamber 7 from connection with a numberof peripheral outlet ports 13. When desired, during operation of therotor, the supply of closing liquid may be interrupted for a short time.Closing chamber 8 is then drained wholly or partly through the channel12, upon which the slide 5 is pressed axially downwardly with referenceto FIG. 1, so that the outlet ports 13 are uncovered.

For the supply of a liquid feed mixture of components to be separated astationary inlet pipe 14 extends into the rotor to a central receivingchamber 15. From the receiving chamber 15 several channels 16 lead intoseparation chamber 7, in which a stack of frusto-conical separationdiscs 17 is arranged.

In the upper part of the separation chamber 7 a conical partition 8 isarranged, which together with the upper rotor part 1 forms a number ofsupply channels 19 for so-called displacement liquid. At its centralportion the partition 18 has two inwardly directed and axially spacedannular flanges 20 and 21. Between these flanges a central outletchamber 23 is formed in the rotor, which communicates with theseparation chamber 7 through an overflow outlet formed by the radiallyinnermost edge portion of the flange 21. Between the flange 20, whichextends somewhat longer radially inwardly than the flange 21, and anupper end wall 24 of the rotor part 1 an inlet chamber 25 is formed fordisplacement liquid. This inlet chamber communicates with the previouslymentioned supply channels 19.

The supply pipe 14 constitutes a part of a surrounding stationary member26, which also forms an inlet channel 27 for displacement liquid and anoutlet channel 28 for a light component of the supplied mixtureseparated in the rotor. The inlet channel 27 opens at 29 in the inletchamber 25, and the outlet channel 28 starts at 30 in the outlet chamber23.

During the operation of the rotor, free liquid surfaces are formed inthe different chambers of the rotors at levels illustrated by means oftriangles in FIG. 1. As can be seen, the part of the stationary member26 forming the outlet channel 28 extends to a level radially outside ofthe free liquid surface in the outlet chamber 23.

The outlet channel 28 is connected to the interior of a conduit 31, inwhich a sensing device 32 is arranged. This sensing device 32 isarranged to detect fractions of displacement liquid accompanyingseparated light liquid out of the rotor. If the light liquid isconstituted by oil and the displacement liquid by water, the sensingdevice 32 may be arranged to sense a change of the dielectric constantof the liquid leaving the rotor.

Through a signal line 33 the sensing equipment 32 is connected with anopening unit 34, which in turn, through a signal line 35, is connectedwith valve means 36 arranged to interrupt the supply of closing liquidto the closing chamber 8 in the rotor, so that its peripheral outlet 13is opened. The sensing device 32 is arranged, immediately upon detectinga predetermined change of the dielectric constant of the liquid flowingthrough the conduit 31, to activate the opening unit 34 so that theperipheral outlet 13 of the rotor is opened during a short period oftime.

The opening unit 34 also is connected through a signal line 37 with acontrol unit 38 arranged to activate the opening unit 34 atpredetermined time intervals so that the peripheral outlet of the rotoris opened for a short period of time.

The control unit 38 is also connected through a signal line 39 to asupply unit 40, which in turn, through a signal line 41, is connectedwith valve means 42. Valve means 42 is situated in a conduit 43, theinterior of which communicates with a pressure source of displacementliquid (not shown) and with the previously mentioned inlet channel 27.The control unit 38 is arranged to activate intermittently the supplyunit 40 to supply displacement liquid to the rotor. Such activationoccurs at a predetermined time in advance of each activation of openingunit 34 which is activated by the control unit 38. The supply unit 40 isadjustable for actuation of the valve 42 to supply displacement liquidto the rotor. According to the present invention the supply unit 40should be adjusted in a way such that when it is activated by thecontrol unit 38, it will first deliver a relatively large and then arelatively small flow of displacement liquid to the rotor. A signal line44 connects the opening unit 34 with the supply unit 40, so thatpossibly current supply of displacement liquid may be interrupted by thesupply unit 40 when the opening unit 34 accomplishes opening of theperipheral outlet 13 of the rotor.

FIG. 1 illustrates by means of dash-dot lines, A, B, C and D fourdifferent radial levels in the rotor separation chamber 7. These levelsare referred to in the following description of the centrifugalseparation operation.

FIG. 2, a and b, illustrate two of several possible methods of supply ofdisplacement liquid within the scope of the present invention.

FIG. 2a illustrates the case where a relatively large portion, P1, ofdisplacement liquid is supplied between two points of time t₁ and t₂ ata speed of L liters per hour, e.g., 60 l/h. During short time intervalsthereafter smaller portions P2-P5 of displacement liquid are supplied atpoints of time t₃, t₄, t₅ and t₆. Each of the last mentioned timeintervals, during which the portions P2-P5 are supplied, is very short,e.g. 5 seconds, whereas the periods of time between the points of timet₂ and t₃ and between subsequent portions P2-P5 preferably are somewhatlonger, e.g., 30 seconds. The average flow rate at which displacementliquid is supplied during the time between the point of time t₂ and thepoint of time when the last portion P5 has been supplied, obviously issubstantially smaller than L l/h.

FIG. 2 illustrates the case where displacement liquid is suppliedbetween the points of time t₁ and t₂ at a rate of L l/h (the same as inFIG. 2a) and between the points of time t₂ and t₇ at a substantiallysmaller rate of S litres per hour.

The following discussion explains how the centrifugal separatoraccording to FIG. 1 works when displacement liquid is supplied in theway illustrated in FIG. 2a. It is assumed that the liquid mixture to betreated is lubricating oil for a diesel motor, which oil is to be freedfrom solids and possibly water. As displacement liquid water is used.

During operating of the rotor, oil feed is supplied continuously throughthe inlet pipe 14, free liquid surfaces being maintained in thedifferent chambers of the rotors as can be seen from FIG. 1. Inconnection with starting the separating operation a small amount ofwater is supplied through the inlet channel 27, the inlet chamber 25 andthe supply channel 19, so that an interface layer between this water andthe oil is formed in the separation chamber 7 at the radial level A. Thereason for the supply of this small amount of water at the beginning ofthe operation derives from the experience that solids to be separatedfrom the oil may more easily be removed from the separation chamberthrough the peripheral outlets 13, if they have been caused to passthrough water. Such an initial supply of water is, however, notabsolutely necessary.

After the separating operation has been going on for a period of timethe length of which is predetermined and entered in the control unit 38,cleaned oil will continuously leave the separation chamber 7 through theoverflow outlet formed by the flange 21. The cleaned oil leaves therotor through the outlet chamber 23, the outlet channel 28 and theconduit 31. During this period of time an unknown amount of solids andan unknown amount of water is separated from the oil flowing through theseparation chamber 7. It is assumed that at the end of the time periodan interface layer between oil and water, or between oil and solids ifno water has been supplied or separated from the oil, is situated atradial level B. At this moment the supply unit 40 is activated by thecontrol unit 38 to start supplying displacement water. In accordancewith FIG. 2a a relatively large portion P1 of displacement liquid isthen supplied between the points of time t₁ and t₂ at a rate of L litersper hour. When the whole portion P1 has been supplied to the separationchamber 7 it is assumed that an interface between oil and water issituated at the level C. Still only cleaned oil flows out of the rotorand passes the sensing equipment 32.

Intermittently the separation chamber is then charged with four smallportions P2-P5 of displacement water, according to FIG. 2a, after whichthe interface between lubricating oil and water in the separationchamber is assumed to have moved to the level D. It is assumed thatsensing equipment 32 neither before nor after the point of time t₆ hassensed any change of the dielectric constant of the oil that has leftthe rotor. When a certain time has passed after t₆ the control unit 38activates the opening unit 34, so that the latter accomplishes openingand closing of the peripheral outlet 13 of the rotor. The opening timeis sufficiently long so that all separated solids and displacement waterare thrown out of the separation chamber 7, but sufficiently short toprevent loss of oil. After this a new separation period is started, thelength of which is determined by the control unit 38.

During the second separation period it is assumed that a somewhat largeramount of water and solids is separated from the oil flowing through theseparation chamber 7 than during the first separation period. It is thusassumed that an interface between oil and water or solids is situated atthe level C in the separation chamber 7 at the time t₁, i.e., when thefirst portion P1 of displacement water starts to be supplied to therotor. After this portion P1 has entered the separation chamber, theinterface layer is situated at the level D, and still only clean oil issensed in the conduit 31.

Even after the portion P2 has been supplied to the rotor only clean oilis sensed in the conduit 31, but some seconds after the portion P3 hasbeen supplied to the rotor, the sensing device 32 detects the existenceof small quantities of water in the cleaned oil, since now the interfacelayer between separated oil and displacement water has reached a levelradially very close to the outer edges of the separation discs 17 andfractions of displacement water are withdrawn by the oil flowingradially inwards between the separation discs. This immediately resultsin activation of the opening unit 34, so that the rotor peripheraloutlet 13 is opened. Simultaneously a signal is given to the supply unit40, so that further supply of displacement liquid is prevented. Thusportions P4 and P5 will not be supplied. The only amount of water whichcan accompany cleaned oil out of the rotor is therefore part of theportion P3.

It has been assumed above that displacement water has been supplied eachtime at the same rate, i.e., L liters per hour. To obtain good controlof the exact amount of supplied displacement water the valve means 42preferably is a so-called constant flow valve, i.e., a valve that in itsopen position always lets through liquid at a predetermined rate,independent of small variations in the pressure drop across the valve.

Within the scope of the invention it is of course possible to add theportion P1 at a certain rate (l/h) and each of the portions P2-P5 at asmaller rate.

According to a further aspect of the invention it is possible toautomatically change the supply of displacement water in connection witha subsequent separation period, if displacement water is sensed in thecleaned oil. Thus, the control unit 38 may be arranged, in connectionwith the subsequent separation period, to adjust the supply unit toreduce the amount of displacement water in the first portion P1 and,instead, to increase the number of small portions, to, for instance,five or six. The current total amount of supplied displacement watershould not be changed. Thus a rapid adaptation to a gradual orunexpectedly large increase of the amount of water or solids in the oilmay be accomplished, so that there will not be too large an amount ofdisplacement water mixed with cleaned oil. If upon a subsequent periodof supply of displacement water no water is sensed in the cleaned oil,the control unit 38 may be arranged to cause the supply unit 40 again tosupply displacement water in the original way.

The control unit 38 also may contain alarm means, which in one way oranother makes an operator aware of the fact that the sensing device 32during subsequent separation periods is forced to initiate opening ofthe rotor periphery outlet as a consequence of sensed displacement waterin the cleaned oil. This may for instance indicate that a water leakagehas come up in the diesel motor through which the lubricating oil iscirculating. The sensing device preferably is arranged to initiateopening of the rotor periphery outlet 13 upon sensing of water in thecleaned oil, even if this happens before displacement water is suppliedat the end of a separation period.

In the above description reference has been made to an opening unit,supply unit and control unit. However, there do not have to be separate"units" for the functions in question. In practice the three units willbe integrated in one single central unit for all functions.

What is claimed is:
 1. In the operation of a centrifugal separatorhaving a rotor, a separation chamber in said rotor, said chamber havingan inlet for liquid feed mixture, a central outlet for separated liquidand a peripheral outlet for separated solids, opening means for openingand closing the peripheral outlet during operation of the rotor, meansfor supplying a predetermined amount of a displacement liquid heavierthan said separated liquid to the separation chamber and control meansfor activating said supply means to supply displacement liquid to saidseparation chamber immediately prior to the opening of said peripheraloutlet, the improvement which comprises:a. supplying a first quantity ofdisplacement liquid to said separation chamber at a first flow rate andthen supplying a second quantity at a substantially lower rate; b.detecting the possible existence of displacement liquid in the separatedliquid leaving the rotor through the central outlet; and c. opening theperipheral outlet in response to the detection of displacement liquid inthe separated liquid.
 2. The method claimed in claim 1, and comprisingsupplying the displacement liquid batch-wise and supplying a subsequentbatch only when the effect of the previous batch has been sensed in theseparated liquid.
 3. The method claimed in claim 1 or 2 and comprisingsupplying the first part of the displacement liquid substantiallycontinuously, and the balance in smaller increments.
 4. The methodclaimed in claim 1 and comprising reducing the size of the firstquantity of displacement liquid in response to the presence ofdisplacement liquid in the separated liquid.
 5. The method according toclaim 1 and comprising interrupting the supply of the predeterminedamount of displacement liquid upon detection of displacement liquid inthe separated liquid.
 6. The method claimed in claim 1 and comprisingopening the peripheral outlet after the predetermined amount ofdisplacement liquid has been supplied, independent of the detection ofdisplacement liquid in the separated liquid.
 7. In a centrifugalseparator comprising a rotor having a separation chamber, said chamberhaving an inlet for a liquid mixture, a central outlet for a separatedliquid and a peripheral outlet for separated solids, opening means foropening and closing the peripheral outlet during operation of the rotor,supply means for supplying a predetermined amount of displacement liquidheavier than said separated liquid to the separation chamber, controlmeans arranged to activate said supply means to supply displacementliquid immediately prior to opening the peripheral outlet, theimprovement which comprises means for regulating said supply means tosupply displacement liquid at a predetermined initial flow rate andafter that at a substantially lower flow rate; a sensing means fordetecting the existence of displacement liquid in separated liquidleaving the rotor through the central outlet and means connected withsaid opening means and said sensing means and arranged to open theperipheral outlet upon the detection displacement liquid in theseparated liquid.
 8. Centrifugal separator according to claim 7 and inwhich the supply means supplies the displacement liquid in an initiallarge batch and then, at predetermined time intervals, in smallerbatches.
 9. Centrifugal separator according to claim 7 and in which thesupply means is connected to the sensing means and arranged to interruptthe supply of displacement liquid when displacement liquid is detectedin the separated liquid.